1. Field of the Invention
The present invention relates to a ignition system for an internal combustion engine.
2. Prior Art
It is the aim of every ignition system to time the spark in the combustion chamber of the cylinders of the engine to provide the maximum torque output. The charge in the cylinder takes a finite time to burn and the spark must ideally be timed to ensure that the termination of the combustion coincides with a particular cranking angle. If the spark is delayed, then the maximum torque is not transmitted to the crankshaft whereas if it is advanced, the pressure developed by the explosion is partly applied prior to top dead center thereby both damaging the engine and reducing the energy transferred to the crankshaft.
Hitherto, several proposals have been made in order to achieve optimum timing, which varies with the numerous parameters affecting the combustion conditions within the cylinder chambers. Thus, the ignition timing needs to be altered in dependence upon speed, upon engine load and upon air fuel ratio among other things.
All existing known ignition systems aim to produce an average timing for the spark which maximizes the torque, this timing angle being referred to herein as MBT (mean best torque) timing. If the timing is occurring correctly at MBT, then the efficiency of the engine will not be determined by the type of ignition system achieving this correct setting. Thus, previously proposed systems for feedback control of the timing in dependence upon various parameters, such as direct measurement of flame propagation speed, have enabled adaptive control of the timing but have not improved engine efficiency as such as compared with a conventional engine where the timing has been accurately set by calibration.
Even when an engine has been correctly tuned, it is noticeable when monitoring the maximum pressure developed within a combustion chamber on consecutive cycles that there are significant variations from one cycle to the next. All that conventional systems can do is ensure that the mean level is optimized. However, the significant fluctuations clearly indicate that with even a correctly tuned engine using known equipment, there are several cycles in which the combustion conditions within the cylinder chamber are different from those predicted, that is to say different from the average, and during all such cycles the internal combustion engine is producing less than optimum output.